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Creep is the tendency of a solid material to slowly deform permanently under the influence of stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature. The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function — for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade.

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Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. Moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.

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The Creep Test: a typical creep curve showing the strain produced as a function of time for a constant stress and temperature. Apply stress to a material at an elevated temperature Creep: Plastic deformation at high temperature

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Flow of vacancies according to (a) Nabarro–Herring and (b) Coble mechanisms, resulting in an increase in the length of the specimen. Diffusion Creep

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Coble creep: a form of diffusion creep, is a mechanism for deformation of crystalline solids. Coble creep occurs through the diffusion of atoms in a material along the grain boundaries, which produces a net flow of material and a sliding of the grain boundaries. Coble creep is named after Robert L. Coble, who first reported his theory of how materials creep over time in 1962 in the Journal of Applied Physics. The strain rate in a material experiencing Coble creep is given by: where σ is the applied stress d is the average grain boundary diameter Dgb is the diffusion coefficient in the grain boundary − QCoble is the activation energy for Coble creep R is the molar gas constant T is the temperature in Kelvin

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Note that in Coble creep, the strain rate   is proportional to the applied stress σ; the same relationship is found for Nabarro-Herring creep. However, the two mechanisms differ in their relationship between the strain rate and grain size d. In Coble creep, the strain rate is proportional to d − 3, whereas the strain rate in Nabarro-Herring creep is proportional to d − 2. Researchers commonly use these relationships to determine which mechanism is dominant in a material; by varying the grain size and measuring how the strain rate is affected, they can determine the value of n in   and conclude whether Coble or Nabarro-Herring creep is dominant.

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Deformation mechanism maps These are graphs in typically stress-temperature space (but also grain size- temperature and others) which show which deformation mechanisms dominate under which conditions

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Deformation mechanism maps These are graphs in typically stress- temperature space (but also grain size- temperature and others) which show which deformation mechanisms dominate under which conditions